The present study examines the distribution of calcium hydroxide, unhydrated cement grain and porosity at the steel-concrete interface. The formation of calcium hydroxide has been confirmed by microscopic analysis using BSE images containing the ITZ between the steel and concrete. It was found that calcium hydroxide does not form a layer on the steel surface, different from the hypothesis that has been available in investigating the corrosion of steel in concrete, ranging from 5 to within the steel surface. Moreover, the high level of porosity at the ITZ was observed, accounting for , which may reduce the buffering capacity of cement hydration products against a local fall in the pH. These findings may imply that the mole of () :() in pore solution as chloride threshold level lead to wrong judgement or to a wide range of values.